1. Technical Field
The present application generally relates to flash memory systems and, more particularly, to the reduction of cross-coupling effects during reading of an addressed memory cell.
2. Related Art
Flash devices may exhibit reading errors due to cross-coupling effects. Cross-coupling effects may be caused by coupling between floating gates of adjacent cells that are in electrical field communication with an addressed cell that is read. The floating-gate-to-floating-gate coupling phenomenon may occur between sets of adjacent memory cells that are programmed at different times. For example, a first memory cell may be programmed to add a level of charge to its floating gate corresponding to one or more bit values. Subsequently, one or more adjacent memory cells may be programmed to add a level of charge to their floating gates corresponding to one or more bit values. After one or more of the adjacent memory cells are programmed, the charge level read from the first memory cell may appear to be different than originally programmed due to cross-coupling effects that the charges on the adjacent memory cells have on the first memory cell. As such, cross-coupling from the adjacent memory cells can shift the apparent charge level read from the first memory cell. This shift may result in an erroneous reading of the data stored in the first memory cell.
Compensation for the cross-coupling between an addressed memory cell and one or more of its adjacent cells may be made when the state of the addressed memory cell is read. Prior methods are based on correction of the read voltage of the addressed cell (or of the estimate of its stored bits) based on the read voltage of the adjacent cells and on the expected voltage shift due to cross-coupling effects from the adjacent cells. These expected voltage shifts are based on the average cross-coupling coefficient, taken over all pairs of adjacent and addressed cell having the same relative location. The problem with this approach is that due to variations in the fabrication process of the flash array, there exist variations in the cross-coupling coefficients between different pairs. These variations result in reduced accuracy of such cross-coupling compensation techniques, which are based on the average value. Variance of these irregularities may increase as manufacturing processes reduce the size of flash memory devices.